Alterations of Fractures in Carbonate Rocks by CO₂-Acidified Brines

Abstract

Fractures in geological formations may enable migration of environmentally relevant fluids, as in leakage of CO2 through caprocks in geologic carbon sequestration. We investigated geochemically induced alterations of fracture geometry in Indiana Limestone specimens. Experiments were the first of their kind, with periodic high-resolution imaging using X-ray computed tomography (xCT) scanning while maintaining high pore pressure (100 bar). We studied two CO2-acidified brines having the same pH (3.3) and comparable thermodynamic disequilibrium but different equilibrated pressures of CO2 (PCO2 values of 12 and 77 bar). High-PCO2 brine has a faster calcite dissolution kinetic rate because of the accelerating effect of carbonic acid. Contrary to expectations, dissolution extents were comparable in the two experiments. However, progressive xCT images revealed extensive channelization for high PCO2, explained by strong positive feedback between ongoing flow and reaction. The pronounced channel increasingly directed flow to a small region of the fracture, which explains why the overall dissolution was lower than expected. Despite this, flow simulations revealed large increases in permeability in the high-PCO2 experiment. This study shows that the permeability evolution of dissolving fractures will be larger for faster-reacting fluids. The overall mechanism is not because more rock dissolves, as would be commonly assumed, but because of accelerated fracture channelization.

Quantifying fracture geometry with X-ray tomography: Technique of Iterative Local Thresholding (TILT) for 3D image segmentation

“…It is also visually evident that before the experiment the fracture was relatively fine, and after the reactive flow experiment, carbonate dissolution had enlarged the fracture. However, the enlargement is not uniform; there are preferential flow pathways separated by regions where the fracture aperture remains very small, as discussed by Deng et al [55]. The close-up images reveal the large variation in fracture aperture and the need for a segmentation algorithm that can characterize both fine and enlarged apertures.…”

“…The fractured core was scanned in this vessel at different stages of the flowthrough experiment. Details about core preparation and experimental conditions are in Deng et al [55].…”

“…The 2D steady-state local cubic law model [21,76] was selected for this purpose, and the same numerical schemes as in Deng et al [55] were used. In the application, there was no smoothing; i.e., the resolution of the flow simulation model was the same as the resolution of the image.…”

“…In contrast, advanced conventional xCT scanner designs, which rely solely on geometric magnification, provide the X-ray flux and source-detector spacing needed to image larger fractured rock cores (e.g., 2 cm dia.) within larger and stronger vessels [55]. The tradeoffs between voxel size, field of view, sample size, and X-ray flux [2,32,33] suggest that both types of instruments will continue to play important roles in core-flow research for the foreseeable future, and regardless of xCT design, larger datasets will continue to accompany advances in instrumentation.…”

“…1 As a testbed for developing TILT, we used two 3D image datasets that exemplify all three challenges listed above. The datasets are from our recently published investigation of fracture geometry alterations due to mineral dissolution caused by reactions with flowing CO 2 -acidified brine [55].…”